* vc/diff-mode.el (diff-mode): Tiny change re diff-default-read-only

Only allow diff-default-read-only to set buffer-read-only to t, never to nil.

Fixes: debbugs:15938

lisp/ChangeLog

 2013-11-23  Ivan Shmakov  <ivan@siamics.net>  (tiny change)
 
+	* vc/diff-mode.el (diff-mode): Only allow diff-default-read-only
+	to set buffer-read-only to t, never to nil.  (Bug#15938)
+
 	* textmodes/tex-mode.el (latex-noindent-environments):
 	Add safe-local-variable property.  (Bug#15936)
 

lisp/vc/diff-mode.el

@@ -1366,7 +1366,8 @@ a diff with \\[diff-reverse-direction].
 
   (diff-setup-whitespace)
 
-  (setq buffer-read-only diff-default-read-only)
+  (if diff-default-read-only
+      (setq buffer-read-only t))
   ;; setup change hooks
   (if (not diff-update-on-the-fly)
       (add-hook 'write-contents-functions 'diff-write-contents-hooks nil t)